1. Field of the Invention
This invention relates generally to protease inhibitors and applications thereof, more specifically to peptide inhibitors of cysteine proteases, even more specifically to aza-peptide epoxides, methods of their use, and methods of their production.
2. Related Art
Protease inhibitors are important therapeutics in the treatment of a variety of disease conditions including viral infections such as HIV infection. Proteases are enzymes that cleave proteins or peptides and are classified into several groups. For example, cysteine proteases form a group of enzymes involved in numerous disease states, and inhibitors of these enzymes can be used therapeutically for the treatment of diseases involving cysteine proteases.
To date, a structurally diverse variety of cysteine protease inhibitors have been identified. Palmer, (1995) J. Med. Chem., 38, 3193, discloses certain vinyl sulfones which act as cysteine protease inhibitors for cathepsins B, L, S, O2 and cruzain. Other classes of compounds, such as aldehydes, nitriles, α-ketocarbonyl compounds, halomethyl ketones, diazomethyl ketones, (acyloxy)methyl ketones, ketomethylsulfonium salts and epoxy succinyl compounds have also been reported to inhibit cysteine proteases. See Palmer, id, and references cited therein. Many irreversible cysteine protease inhibitors have been described in the review by Powers, Asgian, Ekici, and James (2002) Chemical Reviews, 102, 4639. See Powers, id, and references cited therein. However, most of these known inhibitors are not considered suitable for use as therapeutic agents in animals, especially humans, because they suffer from various shortcomings. These shortcomings include lack of selectivity, cytotoxicity, poor solubility, and overly rapid plasma clearance.
In addition, epoxides also have been shown to inhibit cysteine proteases. The first epoxysuccinyl peptide discovered was E-64, a natural inhibitor, which was initially isolated from Aspergillus japonicus by Hanada et al. in 1978. The chemical structure was determined by optical rotation, NMR, IR, MS, elemental analysis, and amino acid analysis to be N-(N-(L-3-trans-carboxyoxiran-2-carbonyl)-L-leucyl)agmatine. Hanada and his coworkers showed that E-64 would inactivate the plant cysteine proteases papain, ficin, and bromelain.

Once the E-64 structure was elucidated, the research groups of Katunuma, Barrett, and others discovered E-64's inhibitory potency toward a large number of other cysteine proteases. E-64 inhibits papain, ficin, bromelain, cathepsin B, H, F, K, L, O, S, V, X, calpain, calpain II, cruzain, and other cysteine proteases. Cathepsin J and streptococcal cysteine protease are slowly inhibited by E-64.
Unlike many other microbial inhibitors, E-64 is a potent and specific irreversible inhibitor of cysteine proteases, and is used as a diagnostic reagent for identification of cysteine proteases. The compound E-64 does not inhibit serine proteases, aspartic proteases, or metalloproteases. However, not all cysteine proteases are inhibited by E-64. Examples of non-inhibited cysteine proteases are legumain and caspases. Caspases and legumain are members of the CD clan of cysteine proteases, while papain, cathepsins, and calpains are members of clan CA. The following table lists those enzymes which are inactivated by E-64 and those which are not inactivated.